Iron and iron-based metal alloys containing alloys such as mild steel are well-known materials used in constructing the apparatus of aqueous systems. In these systems water circulates, contacts the iron based metal surface, and may be concentrated, such as by evaporation of a portion of the water from the system. Even though such metals are readily subject to corrosion in such environments, they are used over other metals due to their strength and availability.
It is known that various materials which are naturally or synthetically occurring in the aqueous systems, especially systems using water derived from natural resources such as seawater, rivers, lakes and the like, attack ferrous-based metals. The term "ferrous-based metals", as used herein, shall mean any iron metal and/or metal alloys containing iron therein. Typical systems in which the iron metal parts are subject to corrosion include evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment and the like. As the water passes through or over the system, a portion of the system water evaporates thereby increasing the concentration of the dissolved materials contained in the system. These materials approach and reach a concentration at which they may cause severe pitting and corrosion which eventually requires replacement of the metal parts. Various corrosion inhibitors have been previously used to treat these systems.
For example, chromates, inorganic phosphates and/or polyphosphates have been used to inhibit the corrosion of metals which are in contact with water. The chromates, though effective, are highly toxic and consequently present handling and disposal problems. While phosphates are non-toxic, due to the limited solubility of calcium phosphate, it is difficult to maintain adequate concentrations of phosphates in many aqueous systems. Polyphosphates are also relatively non-toxic, but tend to hydrolyze to form orthophosphate which in turn, like phosphate itself, can create scale and sludge problems in aqueous systems (e.g. by combining with calcium in the system to form calcium phosphate). Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds can serve as nutrient sources. Borates, nitrates, and nitrites have also been used for corrosion inhibition. These too can serve as nutrients in low concentrations, and/or represent potential health concerns at high concentrations.
Environmental considerations have also recently increased concerns over the discharge of other corrosion inhibiting metals such as zinc, which previously were considered acceptable for water treatment.
Much recent research has concerned development of organic corrosion inhibitors which can reduce reliance on the traditional inorganic inhibitors. Among the organic inhibitors successfully employed are numerous organic phosphonates. These compounds may generally be used without detrimentally interfering with other conventional water treatment additives. However, environmental concerns about the discharge of phosphorus in the form of organic phosphonates have begun to be heard. It is anticipated that in the future this will lead to limitations on the use of organic phosphonates in water treatment.
Another serious problem in industrial aqueous systems, especially in cooling water systems, evaporators, and boilers is the deposition onto heat transfer surfaces of scale, particularly scale-forming salts such as certain carbonates, hydroxides, silicates and sulfates of cations such as calcium and magnesium. Much of the water used in these systems contain various amounts of scale-forming salts. Because of the evaporation which takes place in these aqueous systems, the solids in the water become more concentrated; and, because of the inverse solubility of calcium carbonate, calcium sulfate and other hardness salts, the problem of the formation of water-insoluble scales on the heat transfer surfaces is intensified. In addition, many organic corrosion inhibitors (e.g. hydroxyethylidene diphosphonic acid) are very sensitive to calcium i.e., they have a high tendency to precipitate with calcium ions in solution.
Thus, there is a continuing need for safe and effective water treating agents which can be used to control corrosion, particularly when a substantial concentration of dissolved calcium is present in the system water. Water treating agents of this type are particularly advantageous when they are phosphorus-free.